FIG. 2 schematically depicts a bifacial solar cell, which is one form of conventional solar cells. Moreover, FIG. 3 schematically depicts a mono-facial solar cell, which is another form of the conventional solar cell. In solar cells 100 and 200 depicted in FIGS. 2 and 3, emitter layers 102 and 202 are formed on substrates 101 and 201, respectively. The emitter layers 102 and 202 are formed by thermal diffusion of phosphorus or boron in accordance with the conductivity type of the substrate. Furthermore, back surface field (BSF) layers 103 and 203 are formed by phosphorus diffusion in the case of an n-type substrate and by boron diffusion or alloying of silicon and aluminum in the case of a p-type substrate. In addition, on the emitter layers 102 and 202, passivation layers 104 and 204 are respectively formed, and, also on the back surface field layer 103, the passivation layer 104 is formed. Moreover, the solar cell 100 includes an electrode 105 in contact with the emitter layer 102 and an electrode 106 in contact with the BSF layer 103 on a main surface opposite to a main surface on which the emitter layer 102 is located. On the other hand, the solar cell 200 includes an electrode 205 in contact with the emitter layer 202 and an electrode 206 in contact with the BSF layer 203 on a main surface (a main surface on which the back surface field layer 203 is formed) opposite to a main surface on which the emitter layer 202 is located.
Thermal diffusion of phosphorus is performed by heat treatment at 800° C. to 950° C. using a vapor phase diffusion source such as phosphorus oxychloride or a phosphoric acid-based application-type diffusion source. Moreover, thermal diffusion of boron is performed by heat treatment at 950° C. to 1200° C. using a vapor phase diffusion source such as boron bromide or a boric acid-based application-type diffusion source.
Furthermore, though not depicted in the drawings, when, for example, the diffusion performed on only one surface of the substrate is desired, a thermal oxide film having a film thickness of about 50 nm to 400 nm is sometimes formed on a surface to be prevented from being subjected to the diffusion. In this case, heat treatment at 800° C. to 1100° C. is performed in an atmosphere of oxygen or water vapor.
Moreover, for the substrates 101 and 201, single crystal silicon (CZ-Si) which is obtained by the Czochralski (CZ) process is generally used.
However, when CZ-Si is subjected to the heat treatment, the minority carrier lifetime is often reduced, resulting in low characteristics of the solar cell, which has been a problem. Defects that cause the above low characteristics, which are generally called swirls, are considered to be related to a silicon solid-liquid interface shape on the crystal growth, the concentration of impurities such as oxygen or carbon, and the density of crystal defects such as vacancies because the swirls are concentrically distributed from the central area of the substrate and considered to be generated mainly by oxygen precipitation in a heat treatment process in a subsequent solar cell production process. Thus, in the past, generation of defects has been generally avoided by setting an upper limit on the initial oxygen concentration of a substrate that is used.
On the other hand, for example, Patent Document 1 discloses a method in which a silicon substrate is subjected to heat treatment in an oxygen atmosphere at 1150° C. or higher and then cooling quickly to 950° C. at a rate between 20° C./sec and 5° C./sec, in order to eliminate oxygen precipitation.